The invention relates to an improved method for activating or reactivating potassium nitrate for use in the ion exchange method of strengthening sheet glass.
Glass sheets have previously been strengthened by the well known ion exchange method in which glass sheets are emersed in a molten bath of potassium nitrate. The potassium nitrate, however, must be of a highly purified variety such as that used in the laboratory reagent grades. Untreated commercial grades of potassium nitrate such as NF grade or high "K" technical grade potassium nitrate do not serve to strengthen sheet glass and, additionally, the treatment may result in staining.
In U.S. Pat. No. 3,415,637 a successful commercial method of activating commercial grades of potassium nitrate or reactivating contaminated highly purified grades of potassium nitrate is disclosed. As disclosed in U.S. Pat. No. 3,415,637, potassium nitrate of a commercial grade could be activated or purified by the addition thereto of a small quantity of potassium silicate. The amount of potassium silicate added to the potassium nitrate being dictated by the degree of contamination of the potassium nitrate with impurities, notably lithium. It has now been found that potassium chloride and arsenous sulfide may be advantageously used to activate or purify commercial grades of potassium nitrate.